1. Field of the Invention
The present invention relates to golf balls, particularly a golf ball that exhibits high spinning and that easily comes to a halt even when shot from the rough or when shot in a raining condition such as a balata covered golf ball.
2. Description of the Background Art
Golf balls with a balata cover are generally widely used by low handicap and professional golfers by virtue of its superior hit feeling and controllability. Recently, various soft covers have been proposed to replace balata covers due to their complicated fabrication process and poor cut resistance.
For example, the basic resin of a cover disclosed in Japanese Patent Laying-Open No. 10-179802 is formed having as the main component a heated mixture of ionomer resin, and a styrene-butadiene-styrene block copolymer including a polybutadiene block containing an epoxy group or a styrene-isoprene-styrene block copolymer including a polyisoprene block containing an epoxy group. A golf ball is proposed characterized in that the cover composition forming the cover has a flexural modulus of 50-300 MPa and a Shore D hardness of 40-60.
The basic resin of a cover disclosed in Japanese Patent Laying-Open No. 10-179801 is formed having as the main component a heated mixture of ionomer resin, acid modified thermoplastic elastomer or thermoplastic elastomer with an OH group added at the terminal, and a styrene-butadiene-styrene block copolymer including a polybutadiene block containing epoxy or a styrene-isoprene-styrene block copolymer including a polyisoprene block containing epoxy. A golf ball is proposed characterized in that the cover composition forming the cover has a flexural modulus of 50-300 MPa and a Shore D hardness of 40-60.
Although the golf balls of these inventions are improved in the hit feeling (feeling imparted to the golfer when the ball is hit), controllability, and cut resistance, the spin rate in a wet condition is inferior to the spin rate in a dry condition. These golf balls were not satisfactory from the aspect of spin maintenance.
An object of the present invention is to provide a golf ball having a high ratio of the spin rate in a wet condition to the spin rate in a dry condition, i.e., having a high spin maintenance.
According to an aspect of the present invention, a golf ball formed of a core and a cover is characterized in that the cover composition is blended with basic resin and a tackifier.
Particularly, the basic resin of the cover is preferably ionomer resin alone, or ionomer resin blended with one or at least two types of polystyrene type elastomer, polyolefin type elastomer, polyurethane type elastomer and polyester type elastomer. A tackifier such as terpene resins and rosin ester resin is added 5-50 parts by weight to the basic resin.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.
In the present invention, the ionomer resin employed as the basic resin of the cover is, for example, a copolymer of xcex1-olefin and xcex1, xcex2-unsaturated carboxylic acid of carbon number 3-8, obtained by neutralizing at least a portion of the carboxyl group thereof with metallic ion. Alternatively, a ternary copolymer of xcex1-olefin, xcex1, xcex2-unsaturated carboxylic acid of carbon number 3-8, and xcex1, xcex2-unsaturated carboxylic acid ester of carbon number 2-22, obtained by neutralizing at least a portion of the carboxyl group thereof can be used. As to the composition ratio, 80-90% by weight of xcex1-olefin and 10-20% by weight of xcex1, xcex2-unsaturated carboxylic acid are preferable when the base polymer of the ionomer resin is a copolymer of xcex1-olefin and xcex1, xcex2-unsaturated carboxylic acid of carbon number 3-8. When the base polymer is a ternary copolymer of xcex1-olefin, xcex1, xcex2-unsaturated carboxylic acid of carbon number 3-8, and xcex1, xcex2-unsaturated carboxylic acid ester of carbon number 2-22, 70-85% by weight of xcex1-olefin, 5-20% by weight of xcex1, xcex2-unsaturated carboxylic acid, and 10-25% by weight of xcex1, xcex2-unsaturated carboxylic acid are preferable. It is also preferable that the melt index (MI) of these ionomer resins is 0.1-20, preferably 0.2-20, particularly 0.5-15.
As the xcex1-olefin, ethylene, propylene, 1-butene, 1-pentene, for example, are employed, wherein ethylene is particularly preferable. As the xcex1, xcex2-unsaturated carboxylic acid of carbon number 3-8, acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid, for example, are employed, wherein acrylic acid and methacrylic acid are particularly preferable. As the unsaturated carboxylic acid ester, methyl, ethyl, propyl, n-butyl, isobutyl ester or the like of acrylic acid, methacrylic acid, fumaric acid, maleic acid or the like, for example, are employed, wherein acrylic acid ester and methacrylic acid ester are particularly preferable. As the metallic ion neutralizing at least a portion of the carboxyl group in the copolymer of xcex1-olefin and xcex1, xcex2-unsaturated carboxylic acid or the ternary copolymer of xcex1-olefin, xcex1, xcex2-unsaturated carboxylic acid and xcex1, xcex2-unsaturated carboxylic acid ester, sodium ion, lithium ion, zinc ion, magnesium ion, potassium ion, or the like, for example, can be enumerated. When the ionomer resin has at least a portion of the carboxyl group in the copolymer of ethylene and acrylic acid or methacrylic acid neutralized with metallic ions, the highly rigid and high flow type having a melt index of 3-7 and a flexural modulus of 200-400 MPa is preferable.
Specific trade names of the ionomer resin are enumerated in the following. Hi-milan 1605 (Na), Hi-milan 1707 (Na), Hi-milan AM7318 (Na), Hi-milan 1706 (Zn), Hi-milan AM7315 (Zn), Hi-milan AM7317 (Zn), Hi-milan AM7311 (Mg), and Hi-milan MK7320 (K) sold by Mitsui-Du Pont Polychemical Co., Ltd. are available. As the ternary copolymer ionomer resin, Hi-milan 1856 (Na), Hi-milan 1855 (Zn), Hi-milan AM7316 (Zn), and the like are available from Mitsui-Du Pont Polychemical Co., Ltd. From Du Pont Co. are available Surlyn 8920 (Na), Surlyn 8940 (Na), Surlyn 8945 (Na), Surlyn 9910 (Zn), Surlyn 9945 (Zn), Surlyn 7930 (Li) and Surlyn 7940 (Li) as the ionomer resin, and Surlyn AD8265 (Na), Surlyn AD8269 (Na) and the like as the ternary copolymer type ionomer resin.
As the ionomer resin available from Exxon Chemical Japan Ltd., Iotek 7010 (Zn), Iotek 8000 (Na) and the like can be enumerated. Na, Zn, K, Li, Mg and the like in the above parenthesis following respective trade names of the ionomer resin indicate the metal type of these neutralize metallic ions. In the present invention, the ionomer resin used for the basic resin of the cover may have at least two of those enumerated above mixed. Alternatively, at least two types of the ionomer resin neutralized with monovalent metallic ions enumerated above and ionomer resin neutralized with divalent metallic ions can be mixed to be used.
As the tackifier employed in the present invention, coumarone-indene type resins, polyterpene type resins, rosin derivatives, phenol-formaldehyde type resins, alkyl phenol-formaldehyde type resins, petroleum type resins, xylene-formaldehyde type resins, oligomers of polybutene, liquid rubber such as liquid polyisoprene and the like can be enumerated. Particularly, the terpene type resins and rosin ester type resins are preferably applicable.
Furthermore, hydrogenated terpene resins are favorable. Particularly, hydrogenated terpene resins having a softening point that is not higher than 130xc2x0 C., measured according to JIS-K 6863, are preferable.
Specific trade names of the tackifier are set forth in the following. As the coumarone resins, Process Resin A81, Process Resin AC5, Process Resin TX from Kobe Petrochemical Industry Co., Ltd., Coumarone CL from Ouchishinko Chemical Industrial Co., Ltd., and Coumarone resin NG4 from Nippon Steel Chemical Industries, Co., Ltd. are available. As terpene-phenol resin, Tackirol 101, Tackirol 106, Tackirol EP20, Tackirol EP30 from Sumitomo Chemical Co., Ltd. and Sumilight Resin PR19900 from Sumitomo Dulez Co., Ltd. are available.
As the petroleum type resins, hydrogenated terpene resins from Yasuhara Chemical Co., Ltd., Arkon P90 and Ester Gum H from Arakawa Chemical Co., Ltd., and Petro resin #80, Hilets G100X from Mitsui Petro Chemical Industries, Ltd. are available.
As the rosin derivatives, Nikanol A70 from Mitsubishi Gas Chemical Co., Inc., Lignol R70 from Libnite, and Rosin ester resin from Arakawa Chemical Industries, Co., Ltd. are available.
It is to be noted that a tackifier having a softening point not higher than 130xc2x0 C., measured according to the testing methods for the softening point of hot melt adhesives of JIS-K 6863, is to be used. By using a tackifier having a softening point not higher than 130xc2x0 C., the effect of improving the spin maintenance becomes higher. If the softening point is too low, workability is degraded. Therefore, the preferable range of the softening point for the tackifier is 50xc2x0 C.-130xc2x0 C., further preferably 70xc2x0 C.-110xc2x0 C.
These tackifiers are dispersed-mixed into the basic resin of the cover to provide appropriate adherence to the cover. As a result, the adherence onto the club face is improved when hitting the golf ball. To this end, 5-50 parts by weight, preferably 10-40 parts by weight, further preferably 20-30 parts by weight of the tackifier are blended with respect to 100 parts by weight of the basic resin of the cover. If the blend is less than 5 parts by weight, sufficient adherence cannot be provided to the cover. If the blend exceeds 50 parts by weight, there is a tendency of degradation in the cover""s basic characteristics of hardness, feeling and durability.
As to the basic resin for the cover of the present invention, at least one type of polystyrene type elastomer, polyolefin type elastomer, polyurethane type elastomer and polyester type elastomer can be mixed with ionomer resin for usage.
As specific examples of the polystyrene type elastomer in trade name, ESBS A1010 of Daicel Chemical Industries Ltd., and Septon HG-252 of Kuraray Co., Ltd. can be enumerated.
As specific examples of the polyolefin type elastomer in trade name, Milastomer M4800NW of Mitsui Chemical Industries, Ltd. and Sumitomo TPE 3682, 9455 from Sumitomo Chemical Co., Ltd. can be enumerated.
As specific examples of the polyurethane type elastomer in trade name, Kuramilon 9195, 9180 from Kuraray Co., Ltd. and Elastollan ET880 from Takeda Badische Urethane Industries, Ltd. can be enumerated.
As specific examples of the polyester type elastomer in trade name, Hytrel of Toray-DuPont Co., Ltd. can be enumerated.
The cover composition to form a cover in the present invention can have various additives such as a pigment, dispersion, antioxidant, ultraviolet absorbent, photo-stabilizer and the like added, if necessary.
The foregoing cover composition can be used to cover either a solid core or a thread-wound core.
The solid core can be of the type of one layer structure or a multilayer structure of two or more layers. For example, the solid core of a two piece ball is produced by heating and compressing a rubber composition for 10-40 minutes at the temperature of 140-170xc2x0 C., for example, by press-curing to form into a spherical cured product. The rubber composition is blended with 10-50 parts by weight in total of one or at least two types of a cross-linker formed of xcex1, xcex2-monoethylenic unsaturated carboxylic acid such as acrylic acid or methacrylic acid or metal salt thereof, or a trimethylol propane trimethacrylate polyfunctional monomer, 10-30 parts by weight of a filler such as of zinc oxide or barium sulfate, 0.5-5 parts by weight of a peroxide such as dicumyl peroxide, and, if necessary, 0.1-1 parts by weight of an antioxidant with respect to 100 parts by weight of polybutadiene.
The thread-wound core is constituted by a center and a rubber thread layer formed by winding rubber thread in an elongated state around the center. The center may be a solid center composed of a vulcanizate of a rubber composition, or a liquid center having water or liquid such as paste sealed in the center cover of vulcanized rubber. For the solid center, the diameter is preferably 28-38 mm and the deformation from the initial state of applying an initial load of 10 kg to the center to the state of applying a final load of 30 kg is preferably 0.5-6.0 mm, further preferably 0.5-5.0 mm, and particularly preferably 0.5-2.5 mm. If the diameter of the solid center is smaller than 28 mm, the shot angle will become so small that the spin rate is increased. As a result, the flight distance cannot be increased. If the diameter of the solid center is greater than 38 mm, the winding process of the rubber thread will end before tension is applied on the rubber thread. Therefore, repulsion of the rubber thread layer cannot be exhibited sufficiently, so that the impact resilience of the ball is degraded. As a result, the flight distance cannot be increased. If the deformation of the solid center is a smaller than 0.5 mm, the center will be too hard for the ball. The hit feeling when the ball is shot will be degraded. If the deformation of the solid center is greater than 6.0 mm, the center will become too soft. The appropriate hardness for the ball cannot be obtained. There is also the possibility that the ball""s initial speed is decreased. For a liquid center, the diameter is preferably 26-34 mm. If the diameter of the liquid center is smaller than 26 mm, the shot angle will be reduced and the spin rate increased. As a result, the flight distance cannot be increased. If the diameter of the liquid center is greater than 34 mm, the predetermined ball hardness cannot be obtained. Also, the impact resilience of the ball will be degraded since the rubber thread layer becomes thinner. As a result, the flight distance cannot be increased.
The foregoing rubber thread can be obtained by vulcanizing a rubber composition having an antioxidant, vulcanization accelerator, sulfur and the like blended with natural rubber or blended rubber of natural rubber and synthesized polyisoprene.
The cover of the golf ball of the present invention has a Shore D hardness within the range of 40-65 after molding. If the Shore D hardness of the cover composition is lower than 40, the cover will become so soft that the cut resistance is degraded. If the Shore D hardness is greater than 65, an appropriate back spin rate cannot be obtained. As a result, the controllability and hit feeling are degraded.